1. Field of the Invention
Steam turbines whose design includes double shell construction require devices that allow the two shells to expand and contract differentially, without allowing significant leakage out of the steam pipes that carry steam from the outer shell to the inner shell.
2. Description of the Prior Art
A common system employed by turbine manufacturers is called a bell seal. The bell slides into a tube held by the inner shell providing a minimum radial clearance with the tube, yet allowing vertical differential motion of the inner and outer shells. The bell is also secured to a tube held by the outer shell in such a way that it can slide, permitting differential motion in either the lateral or axial directions relative to the shaft, yet maintaining a small clearance that keeps leakage to a minimum.
This bell seal system is commonly found, after service, to have a clearance between the bell and the inner shell tube of about 0.010 inches. This allows significant leakage and loss of turbine output. Replacement of the bell seal is usually ineffective, with the clearance and leakage recurring.
The major problem is that the bell seal itself is of very powerful construction and when it becomes hot during starting procedures, while the inner shell tube is still relatively cool, its thermal growth can stretch and crush the opposing surfaces on the inner shell tube. Even during steady state operation, the bell may be hotter than the inner shell tube. The described problem is especially apparent on larger turbines where the bell diameter is greater.
An improvement to the bell seal system that prevents crushing the mating surfaces or stretching them beyond the elastic limit would provide significant improvement in turbine efficiency.